On metal scraping (this is blog post #400, BTW)

A deeper, more thorough treatise on the subject
of hand scraping may be found here:
https://qdmetrology.com/scraping-for-precision-machine-tools/

 Hand scraping is a technique for removing very small amounts of material from metal surfaces, almost exclusively various irons - either bearing surfaces on machines or reference surfaces on things like straightedges and iron surface plates.  Scraping makes a nicely ground surface uglier... but better.  It is used to accomplish two things: to make the surface even flatter than can be achieved with a surface grinder, and to improve oil retention on the surface, which is super important on machine ways.

fig. Uno
This is oil pattern scraping, performed as the finishing step.
Scraping done in the roughing step appears less regular.

 The amount removed is typically in the range of 0.0002" to 0.001" (about 5 to 25 microns) depending on the operator's technique and whether they are at the roughing or the finishing stage. 

 The amount of material removed is not uniform across the surface; rather, the process targets and removes only the microscopic high spots, which are identified by lightly pressing the work against a reference surface which has been coated with a very thin layer of a marking compound, typically Prussian Blue.  This is called "printing" the work. 

 

 The high spots pick up the marking compound, the low spots don't, and you scrape away the blue parts and repeat until it shows lots of small light marks of roughly equal size and shape, all over.

 Fig. 1 shows a nice job of Moore pattern scraping, applied from several directions.  This is on the saddle ways on the knee of my Bridgeport it is ≈50 yrs old on a machine heavily used in a tool & die shop.  Oil retention matters!

fig. OMG NO

  And, looking out of the left side of the bus, we have the disastrous "scraping" that was done on the cross-slide of my '70s Chinesium lathe.  It is to cringe.  Would love to have met the pendejo responsible, (so I could punch him in the nose) but he's long dead, what can ya do?

 The marks are random and VERY deep, and it's not clear whether they were made with a power scraper  with a dull blade... or a cold chisel!  Those gouges along the edge... I dunno man, I just don't know.  Es mui mal.

 Either way, they are unreasonably bad and the ways seem to be as flat as the Devil's Golf Course, but I haven't got a reference straightedge nor the scratch to buy one in order to verify that belief.

 I think I'm at the point of actively wanting a roughly 18in "master" straightedge.  All scraped and ready to go, they are fantastically expensive - a few hundred for a suspect Chinesium one (they are that labor intensive!), north of six hundred simoleons for a Freedom Product.  Sadly, Charlie's Secondhand Tools here in Denver is closing forever even as I write this, so I'll have no chance of lucking into one there.  I don't patronize the flea market, it's all stolen goods.  I'll have to keep an eye out for property disposal auctions and such like, and of course, eBay, cuz ya never know.

 There are folks who sell straightedge castings, fully stress-relieved, for you to grind, lap, and/or scrape to your heart's delight.  And even those are kinda spendy because they're cast in small batches, then there's labor and electricity for the heat soak.  The iron is thrown in for free.  ;)  Nevertheless, that might be my best option if I want to possess one before I'm compost.

 There is an old colloquial expression for what I'm attempting to do with this lathe; "making a silk purse out of a sow's ear".  The inference is that it can't be done, because of what you started with. :/